The landmark sedition trial of five members of the far-right Oath Keepers has opened with US prosecutors telling a jury that the group heavily armed itself on 6 January 2021 to attack the US Capitol to keep Donald Trump in the presidency.

Justice Department attorney Jeffrey Nestler said that Stewart Rhodes, the eyepatch-wearing former soldier and Yale law school graduate, knew exactly what he was doing when he led the militia's followers towards the Capitol.

Showing videos of the violent assault by dozens of group members dressed in military-style combat gear, Mr Nestler said Mr Rhodes directed them "like a general on the battlefield" as they sought to prevent 2020 election winner Joe Biden from being certified as the next president.

On 6 January the Oath Keepers "concocted a plan for an armed rebellion ... plotting to oppose by force the government of the United States," Mr Nestler said.

"They did not go to the capital to defend or to help. They went to attack," he said.

Trump supporters breaking into the US Capitol

'Peacekeeping force'

But Mr Rhodes' lawyer Phillip Linder, the first to present for the defence, rejected the government case, saying Mr Rhodes had brought the Oath Keepers to Washington to provide security for Mr Trump's speech that day and other pro-Trump events.

"The Oath Keepers are basically a peacekeeping force," said Mr Linder.

"The real evidence is going to show our clients were there to do security for events that were scheduled for the 5th and 6th" of January 2021, he said.

They had created an armed "quick reaction force" on that day just in case they were needed - it would have been "defensive", he said, "if Mr Trump called them in.

"Stewart Rhodes did not have any violent intent that day," he said.

Rare charge

Mr Nestler's presentation was the opening argument for the case, which sees Mr Rhodes, and four other Oath Keep leaders Kelly Meggs, Thomas Caldwell, Jessica Watkins, Kenneth Harrelson, facing the rarely used Civil War-era charge of seditious conspiracy.

With a potential 20-year prison sentence, the charge is the toughest yet in the prosecutions of hundreds who took part in the Capitol assault, which aimed to reverse President Joe Biden's victory in the November 2020 election.

Of the some 870 people charged in the riot, which saw Congress evacuated and dozens of police officers injured by attackers, most are accused of lesser crimes like assault and disrupting an official government meeting, as well as illegal entry.

Trump supporters stormed the US Capitol on 6 January 2021

The US government has reserved sedition for just a few dozen of the attackers, mostly members of self-styled militia groups like the Oath Keepers and Proud Boys who allegedly planned and coordinated the attack.

The Oath Keepers' attorneys are expected to argue to the jury that they believed Mr Trump would invoke the 1807 "Insurrection Act," deputising them to protect the country.

That claim has raised expectations that the trial could reveal more about links between the Capitol attack and members of Mr Trump's administration or his personal advisers.

But, in anticipation of that argument, Mr Nestler said it was something Mr Rhodes, graduate of one of the country's most prestigious law schools, concocted as a legal strategy to protect their actions.

"[Mr] Rhodes' talk about the Insurrection Act was legal cover," he said.

Despite being called on by many supporters to invoke it in the weeks and days before 6 January, Mr Trump never did, he noted.

Trump supporters marched on the US Capitol after his speech

Instead, Mr Rhodes and the others spoke on encrypted chats of launching a civil war to prevent Joe Biden from becoming president.

"I'm personally gonna start the civil war myself," if Congress moves to certify Mr Biden as president-elect, Mr Caldwell wrote to the others, the jury was told.